Technical Abstract:
The objective of this research was to determine whether daughter pregnancy rates (DPR) and estimated relative conception rates (ERCR) have a bimodal distribution for certain bulls as seen with perinatal survival. Perinatal survival rates were computed for 12,034 sons at Iowa State University. AIPL calculated DPRs for 48,117 sons. One hundred and seventy six bulls with a minimum of 50 sons had DPRs. Dairy Records Management Systems computed ERCRs for 3,276 sons from breedings in 1988 through 1997. Seventeen bulls had at least 50 sons with ERCRs and were included in the analysis. A kernel density procedure estimated density of DPR and ERCR for each bull. Three different bandwidth multipliers were used (1,2,3). Densities that were bimodal at bandwidth multiplier one but fail to remain bimodal at bandwidth multiplier two were considered unimodal. None of the bulls were bimodal for DPR or ERCR. Of the 21 bulls that were bimodal for perinatal survival, all 21 had DPRs but only five had ERCRs. A significant (P < 0.0001) correlation of 0.134 between DPR and perinatal survival (8552 sons) was found. This positive correlation indicates that bulls with high DPR also have high perinatal survival. The correlations between DPR with ERCR for 1996 (1346 sons) and DPR with ERCR for 1997 (1611 sons) were not significant, nor were the correlations between perinatal survival with ERCR for 1996 (463 sons) and perinatal survival with ERCR for 1997 (414 sons). A significant (P < 0.0001) correlation of 0.361 between ERCR for 1996 and ERCR for 1997 (541 sons) was found. From this research we conclude that daughter pregnancy rates and estimated relative conception rates do not have a bimodal distribution like the ones seen with perinatal survival.